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The acid-assisted wet-chemical and the adhesive-tape induced micromechanical exfoliation of differently sized single crystals of a 2D polymer (approx. 20 μm and 100 μm) is shown to result in thin sheet stacks. Tuning of the thickness is achieved via duration and frequency of the exfoliation, respectively. A color code is established that correlates interference colors of sheet stacks on SiO 2 (300 nm)/Si as observed under an optical microscope with their thicknesses measured by atomic force microscopy. This facilitates reliable monitoring of the exfoliation and quick identification of sheet stacks of a desired thickness. Furthermore, high resolution atomic force microscopy is applied to investigate the surfaces of starting crystals and both wet-chemically and micromechanically exfoliated sheet stacks aiming at exploring whether exfoliation proceeds with preservation of surface periodicity and with a low frequency of sheet rupturing. These investigations also aimed at uncovering possible point defects and domain (grain) boundaries in the surfaces. It appears that all investigated objects have a high molecular scale perfection and that both exfoliation methods proceed mild enough to largely preserve the molecular structure of the 2D polymer including the not covalently bonded template molecules being part of the crystal packing.

This work proves the existence and chemical addressability of defined edge groups of a 2D polymer. Pseudohexagonally prismatic single crystals consisting of layered stacks of a 2D polymer are used. They should expose anthracene-based edge groups at the six (100) but not at the two pseudohexagonal (001) and (001̅) faces. The crystals are reacted with the isotopically enriched dienophiles maleic anhydride and a C18-alkyl chain-modified maleimide. In both cases the corresponding Diels-Alder adducts between these reagents and the edge groups are formed as confirmed by solid state NMR spectroscopy. The same applies to a maleimide derivative carrying a BODIPY dye which was chosen for its fluorescence to be out of the range of the self-fluorescence of the 2D polymer crystals stemming from contained template molecules. If the crystals are excited at λ = 633 nm, their (100) faces and thus their rims fluoresce brightly, while the pseudohexagonal faces remain silent. This is visible when the crystals lie on a pseudohexagonal face. Lambda-mode laser scanning microscopy confirms this fluorescence to originate from the BODIPY dye. Micromechanical exfoliation of the dye-modified crystals results in thinner sheet packages which still exhibit BODIPY fluorescence right at the rim of these packages. This work establishes the chemical nature of the edge groups of a 2D polymer and is also the first implementation of an edge group modification similar to end group modifications of linear polymers.

Two-dimensional materials have moved into the spotlight of researchers. The isolation of single graphene sheets has shown that restricted dimensionality can lead to interesting properties. Bottom-up synthesis of organic, covalently-bonded structures is, however, still challenging. In this publication we give a synopsis of a recently published monomer that is easily accessible and reversibly provides chiral two-dimensional polymer single crystals, which can be exfoliated into nanometer-thin sheets and show promise for applications in, for example, nonlinear optics and ultrathin membranes.

The rise of graphene, a natural two-dimensional polymer (2DP) with topologically planar repeat units, has challenged synthetic chemistry, and has highlighted that accessing equivalent covalently bonded sheet-like macromolecules has, until recently, not been achieved. Here we show that non-centrosymmetric, enantiomorphic single crystals of a simple-to-make monomer can be photochemically converted into chiral 2DP crystals and cleanly reversed back to the monomer. X-ray diffraction established unequivocal structural proof for this synthetic 2DP, which has an all-carbon scaffold and can be synthesized on the gram scale. The monomer crystals are highly robust, can be easily grown to sizes greater than 1 mm and the resulting 2DP crystals exfoliated into nanometre-thin sheets. This unique combination of features suggests that these 2DPs could find use in membranes and nonlinear optics.

The facile synthesis of a rotor-shaped compound with two stacked triazine units, which are symmetrically connected by three anthracene blades through oxygen linkers, is presented. This new double-decker, which is a potential monomer for two-dimensional polymerization, was synthesized by using readily available, cheap building blocks, exploiting the known selectivity difference for the nucleophilic substitution of cyanuric chloride. The crystal structure of a C3h symmetric rotor-shaped compound with 9,10-dihydroanthracene blades, which is a direct precursor to the targeted monomer, and the crystal structure of the new double-decker with the desired C3h symmetry, are also reported. The synthetic efforts were preceded by a computational analysis, which was triggered by the question of conformational stability of the potential monomer. Two stable conformers could be found, and the barrier for the transition path in the gas phase between these conformers was determined by quantum chemical calculations. Exploratory Born-Oppenheimer molecular-dynamics simulations revealed a strong influence of solvent-solute interactions on the stability of the conformers, which resulted in an energetic preference of the C3h symmetric conformation of the double-decker., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)